The present invention relates to electric cigar lighter heating elements of the type disclosed and claimed in Mattis U.S. Pat. No. 3,958,099. In the past, covering a considerable period of years, the design and manufacture of heating element units for cigar lighters have undergone change not so much in the form of substantial innovations as in the form of relatively minor improvements directed toward enhancing the quality of the product and reducing its cost.
For many years a standardized form of cigar lighter heating element consisted of a flat ribbon which was wound into a pancake spiral and then supported in a metal cup, with the outermost convolution of the ribbon being welded to the inside surfaces of the cup wall. The cups were uniformly made of steel stampings, and to overcome problems of poor contact, abrasion, etc. where the rims of the cups were latched onto by the bimetallic circuitbreaking fingers, the cups were plated as with nickel, chrome, gold, silver and the like.
For the purpose of combating abrasion, the cups were case hardened with a deeply penetrating nitriding process, as by heating them in an atmosphere of ammonia gas. Thereafter, the cups were cleaned as by tumbling, and then plated with nickel, silver or other metal to prevent rusting and corrosion, especially at the contacting outer rim surfaces where engagement occurred with the bimetallic latching fingers.
The above case hardening procedure which was followed by nickel plating found wide acceptance. Previously, the use of gold plating or silver plating resulted ultimately in the wearing away of the plated metal and a consequent rusting and deterioration of the cup surface, even though no appreciable deep abrasion occurred because of the case hardening treatment.
One disadvantage of the deep case hardening involved possible distortion of the stamped and drawn cup, which could result in malfunctioning and required careful checking to assure uniformity in the end product.
The various changes in the design and manufacture of heating element cups did not occur quickly, but instead the improvements were made over the years as a result of extensive testing and inspection after thousands of operations of the cigar lighter in the intended manner.
Recently an important innovation has been achieved in the manufacture of heating elements for cigar lighters and the like, as set forth in recently issued U.S. Pat. No. 3,958,099 dated May 18, 1976 and entitled "Electrical Resistance Unit For Ignitor Plugs", the inventor being Donald J. Mattis. This innovation enabled the welding procedure to be eliminated completely, and further eliminated the sharp and irregular cut edge of the steel heating element cup, providing instead a uniform and attractive rolled-in edge which resulted in a highly desirable finished appearance with no sharp edges. The rounded or rolled-in edge was done in such a manner that it crimped and securely held the outermost end of the spiral heating ribbon contained in the cup. The rolling-in of the edge not only eliminated the costly welding procedure with its consequent inspection and spot-checking, but it also enabled an automated assembly of the heating element to the cup to be effected, in a quicker and more economical manner. At the same time, the rolled-in edge provided an improved electrical connection which did not require subsequent inspection, spot-checking, etc.
In spite of the many advantages of the rolled-in edge which eliminated the welding there remained a problem with respect to the finish to be provided on the metal cup. It was not found possible to employ the usual case hardening and nickel plating for the reason that the cup wall would be too brittle, resulting in objectionable cracking while at the same time there was a strong tendency for the nickel plating to peel away during the curling-in process of the cup rim. In the environment used, the electrical heating elements require resistance to abrasion and rusting while also effecting a positive electrical contact capable of carrying the required electrical amperage. It was not immediately apparent how the problem could be solved without resorting to additional plating operations or the like which defeated the objective of providing a more economical unit and one which would lend itself to an automated assembly.